Long life lubricating oil composition using particular antioxidant components

ABSTRACT

A natural gas engine oil composition with enhanced resistance to oxidation, nitration and viscosity increase comprising: an oil basestock; neutral and overbased metallic detergents; an antioxidant comprising an alkylthiocarbamoyl compound or a mixture of an alkylthiocarbamoyl compound and an ashless, non-sulfur containing, hindered phenol; and a zinc dihydrocarbyl dithiophosphate in an amount sufficient to provide the composition with from 250 to 450 wppm phosphorus.

This application claims the benefit of U.S. Ser. No. 60/500,378 filed Sep. 5, 2003.

FIELD OF INVENTION

The present invention relates to gas engine oils. More particularly the present invention relates to gas engine oils that provide enhanced resistance to oxidation, nitration and viscosity increase.

BACKGROUND OF INVENTION

Typical natural gas fired engines such as those used in the petroleum industry to compress natural gas at well heads and along pipelines have up to 16 cylinders, often generating between 500 to 3000 HP. These engines normally are run continuously near full load conditions with shut downs being primarily for maintenance such as for oil changes. Such continuous operation near full load, of course, places severe demands on the engine lubricant. Indeed, because the lubricant is subjected to a high temperature environment, oxidation processes can occur rapidly which limit lubricant life. Also, natural gas engines emit nitrogen oxides (NO_(x)), some of which come into contact with the lubricant resulting in nitration processes also limiting lubricant life. Typically these processes are accompanied by increases in oil viscosity.

Thus, it is desirable to extend the life of gas engine oils by enhancing the oil's resistance to oxidation and nitration and to reduce viscosity increases in the oil.

To extend lubricant life, base oils are formulated with various additives such as dispersants, detergents, antioxidants, viscosity index improvers and the like to provide a lubricating oil composition. This art of lubricating oil formulation, however, has become increasingly more complex with ever more stringent requirements by end-users. Indeed, experience has shown that incorporation of one type of additive in a lubricant composition can have a negative impact on the function of another type of additive. Consequently, extensive research continues in the quest for lubricants of improved life and function.

An object of the present invention is to provide a gas engine lubricating composition that has enhanced resistance to oxidation and nitration.

Another object of the invention is to provide a gas engine lubricating composition that has improved life as evidenced by reduction in viscosity increase.

SUMMARY OF INVENTION

Accordingly, a natural gas engine lubricant composition having enhanced resistance to oxidation, nitration and viscosity increase comprises:

-   -   (a) a major amount of an oil basestock of lubricating viscosity         having a kinematic viscosity at 100° C. of about 5 to 16 cSt and         preferably about 10 to about 13 cSt;     -   (b) a minor amount of a detergent mixture comprising low/neutral         TBN and over based metallic detergents, selected from the group         consisting of alkali and alkaline earth sulphonates, phenates,         and salicylates;     -   (c) a minor amount of an antioxidant comprising an         alkylthiocarbamoyl compound or a mixture of an         alkylthiocarbamoyl compound and an ashless, non-sulfur         containing hindered phenol; and,     -   (d) a zinc dihydrocarbyl dithiophosphate in an amount sufficient         to provide the lubricant composition with from 250 to 450 wppm         of phosphorus.

Other gas engine oil additives also may be present. These include: ashless dispersants, metal passivators, pour point depressants, VI improvers and antifoamants.

DETAILED DESCRIPTION OF THE INVENTION

The lubricating oil composition of the invention comprises a major amount of a lubricating oil basestock which may be a mineral oil, synthetic oil or blends of oils to give a basestock of the desired viscosity for a natural gas engine oil. Typically, the basestock of the invention will have a kinematic viscosity at 100° C. in the range of about 5 to about 16 cSt and preferably from about 10 to about 13 cSt. Especially preferred are API category Group II basestocks.

The composition of the invention includes a mixture of low/neutral TBN and overbased (high TBN) metallic detergents selected from the groups consisting of alkali and alkaline earth metal sulphonates, phenates, and salicylates. Preferably the metallic detergents will be calcium sulphonates, calcium phenates and calcium salicylates. Low/neutral TBN metallic detergents typically have a TBN in the range of about 10 to about 80. High TBN metallic detergents typically have a TBN in the range of about 150 to 300 or higher.

The metallic detergents are used in amounts sufficient to contribute a sulfated ash (ASTM D-874) to the formulated lubricant oil composition of about 0.2 mass % to about 2.0 mass %. Expressed in terms based on active ingredient in the detergent mixture, the metallic detergents comprise from about 0.5 volume % to about 10 volume % and preferably 0.5 volume % to 5.0 volume % of the lubricating composition. The volume ratio (based on active ingredient) of overbased to low/neutral TBN metallic detergents is in the range of from about 0.05 to 3.5, and preferably about 0.25 to 1.0.

The lubricating composition of the invention contains a minor but effective amount of an antioxidant comprising an alkylthiocarbamoyl compound or a mixture of an alkylthiocarbamoyl compound and an ashless, non-sulfur containing, hindered phenol.

Suitable alkylthiocarbamoyl compounds are represented by the formula:

where R¹, R², R³ and R⁴ are the same or different linear and branched alkyl groups of from 1 to 18 carbon atoms, X is S, S—S, S—(CH₂)_(y)—S, S—CH₂—CH(R⁵)—S;

-   y is an integer of 1 to 4, and R⁵ is an alkyl group of 1 to 2     carbons. Preferably R¹, R¹, R³ and R⁴ are C₄ alkyl groups, X is     S(CH₂)_(y)S, Y is 1-3.

The alkylthiocarbamoyl compound when used in the absence of the ashless, non-sulfur containing, hindered phenol generally comprises from about 0.25 volume % to about 2.0 volume % and preferably 0.5 volume % to 1.5 volume % of the total volume of the lubricating oil composition.

In an alternate embodiment, the composition of the invention contains an antioxidant comprising a mixture of an alkylthiocarbamoyl compound and an ashless, sulfur free, hindered phenol. In this embodiment, suitable alkylthiocarbamoyl compounds are represented by the same formula above. The hindered phenols suitable for use in conjunction with the alkylthiocarbamoyl compounds may be represented by the following formulae:

where R₁, R₂ and R₃ are the same or different alkyl groups of 1 to 18 carbon atoms.

In the embodiment where the antioxidant is a mixture of alkylthiocarbamoyl and ashless, non-sulfur containing hindered phenols, the alkyl dithiocarbamoyl comprises 0.25 volume % to 1.5 volume %, and preferably 0.5 volume % to 1.0 volume % of the composition and the phenol from 0.25 volume % to 1.5 volume % and preferably 0.5 volume % to 1.0 volume %.

The natural gas engine lubricating composition also includes a zinc dihydrocarbyl dithiophosphate (ZDDP) or mixture of ZDDPs in which the hydrocarbyl groups may be the same or different alkyl groups or alkylaryl groups with the alkyl group, in each instance, having 3 to about 18 carbon atoms. Preferably the hydrocarbyl group is an alkyl group. The ZDDP or ZDDP mixture is present in the amount sufficient to provide the lubricating oil composition with 250 to 450 wppm of phosphorus.

The invention also contemplates the use of other gas engine oil additives such as ashless dispersants, metal passivators, pour point depressants, VI improvers and antifoamants. Some of these additives are only required in minor amounts (antifoamants, metal passivators), while others may be required in significant amounts (dispersants). Thus, these additives will typically range from 0.01 volume % to about 10 volume % based on the total volume of the engine oil composition.

EXAMPLES AND COMPARATIVE EXAMPLES

The invention will be further illustrated by the following Examples and Comparative Examples.

Table 1 below details a series of formulations: (a) two for particularly preferred embodiments of the invention and (b) three other formulations included for comparative purposes. The basestock in all formulations was an API Group II category basestock. All oils were SAE 40 grades with nominally 0.45 mass % sulphated ash.

Comparative Oil 1 is a low ash commercial gas engine oil which employed a commercial gas engine oil additive package.

Reference Oil 1 was blended to represent an oil of U.S. Pat. No. 6,140,282, while Comparative Oil 2 was blended to represent an oil of U.S. Pat. No. 5,569,405.

The various formulations were subjected to a nitration screener test and the results are presented in Table 1. The nitration screener test is a lab test which assesses several facets of the degradation of natural gas engine oils. All results are expressed as normalised against the results for the Reference Oil. Therefore, all results for the Reference Oil will have a result of 1.00 and any results lower than 1.00 signify enhanced performance. TABLE 1 Reference Comparative Oil 1 Oil 2 Based on Based on Comparative U.S. Pat. No. Invention U.S. Pat. No. Invention Formulation Description. 

Oil 1 6,140,282 Example 1 5,569,405 Example 2 Component (vol %) Group II Group II Group II Group II Group II

Basestock Description 

basestocks basestocks basestocks basestocks basestocks Group II basestock 87.90 90.00 90.00 90.00 90.00 NGEO Commercial Additive package 9.60 — — — — VII 1.00 1.00 1.00 1.00 1.00 PPD 0.50 — — — — Calcium alkylsalicylate, 64 TBN — 2.00 2.00 2.00 2.00 Calcium alkylsalicylate, 280 TBN — 0.40 0.40 0.40 0.40 Balance of Additive System — 5.60 5.60 5.60 5.60 Phenolic antioxidant 1.00 1.00 — — 0.50 Sulphur-containing Phenolic antioxidant — — — 0.50 — Ashless alkylthiocarbamoyl — — 1.00 0.50 0.50 Kinematic viscosity, cSt measured kV @ 100° C. 13.25 13.21 13.15 13.12 13.19 Phosphorus content, ppm 334 295 295 295 295 Nitration Screener Test oxidation (relative to reference Oil 1) 2.92 1.00 1.07 1.37 0.83 nitration (relative to reference Oil 1) 1.59 1.00 0.79 0.73 0.59 viscosity increase (relative to Reference 2.80 1.00 −0.43 0.21 0.17 Oil 1) PPD = pour point depressant

As can be seen, Comparative Oil 1 is inferior in all respects to the Reference Oil while Comparative Oil 2 provides better nitration and thickening control than the Reference Oil but inferior oxidation control. The Example 1 oil in contrast provides roughly equivalent oxidation control to that of the Reference Oil and improved nitration and thickening control. The oil of Example 2 is better than the Reference Oil in all three parameters. 

1. A natural gas engine lubricating oil composition having enhanced resistance to oxidation, nitration and viscosity increase comprising: a major amount of an oil of lubricating viscosity; a minor amount of a detergent mixture comprising low/neutral TBN and over based metallic detergents selected from the group consisting of alkali and alkaline earth sulphonates, phenates, and salicylates; a minor amount of an antioxidant comprising an alkylthiocarbamoyl compound or a mixture of an alkylthiocarbamoyl compound and an ashless, non-sulfur containing hindered phenol; and a zinc dihydrocarbyl dithiophosphate in an amount sufficient to provide the composition with from 250 to 450 wppm phosphorus.
 2. The composition of claim 1 wherein the oil basestock has a kinematic viscosity at 100° C. of about 5 to about 16 cSt.
 3. The composition of claim 2 wherein, based on the total volume of the composition the detergents comprise from about 0.5 volume % to about 10 volume %.
 4. The composition of claim 3 wherein the antioxidant is an alkylthiocarbamoyl compound which comprises about 0.25 volume % to about 2.0 volume % of the composition.
 5. The composition of claim 3 wherein the antioxidant is a mixture of an alkylthiocarbamoyl compound and an ashless, non-sulfur containing hindered phenol, wherein the alkylthiocarbamoyl compound comprises 0.25 volume % to 1.5 volume % of the composition and the phenol, 0.25 volume % to 1.5 volume % of the composition.
 6. The composition of claim 4 or 5 wherein the alkylthiocarbamoyl compound is represented by the formula

where R¹, R², R³ and R⁴ are the same or different linear and branched alkyl groups of from 1 to 18 carbon atoms, X is S, S—S, S—(CH₂)_(y)—S, S—CH₂—CH(R⁵)—S; y is an integer of 1 to 4, and R⁵ is an alkyl group of 1 to 2 carbons. Preferably R¹, R², R³ and R⁴ are C₄ alkyl, X is S(CH₂)_(y)S, Y is 1-3.
 7. The composition of claim 5 wherein the ashless, non-sulfur containing, hindered phenol is represented by the formulae:

where R₁, R₂ and R₃ are the same or different alkyl groups of 1 to 18 carbon atoms. 